The hydraulic axle combinations of the present invention are, for example, utilized in powered, wheeled, drive units that typically find utility in known, zero-turn-radius (ZTR) vehicles. Such a ZTR vehicle often takes the form of a grass or lawn mowing machine having at least one cutting blade. A ZTR vehicle utilizes a single prime mover or engine that drives a separate, independent transmission for each of its left and right side rear driving wheels. Independent front left and right side wheels are usually caster-type wheels that are free to pivot about their vertical clevis axes, in a manner well known in the art. Today, most commercial ZTR machines are driven by separate, dual, hydrostatic transmissions, for left and right driven wheels. Each such transmission preferably includes its own closed-loop circuit and both transmissions function independently and can, for example, be operated or controlled via their human operator-actuated right and left control levers. In one such mode of operation, each of control levers is linked to a respective trunnion shaft of an associated hydraulic pump and enables the operator to control the direction and amount of fluid flow from each transmission pump. When the operator pushes a lever in one direction, the associated pump delivers fluid flow in one direction of corresponding motor operation. When the operator pushes the lever in the opposite direction, the associated pump delivers fluid flow in the opposite direction of corresponding motor operation. Thus, the use of dual hydrostatic transmissions permits the zero-turn-radius features of these vehicles, which is an important feature, particularly in commercial mowers.
The patent literature includes a large number of designs and structures pertaining to hydraulic axle combinations and hydraulic transaxles, including but not limited to the following: U.S. Pat. No. 2,780,424 to Price; U.S. Pat. No. 2,996,135 to Grabow; U.S. Pat. No. 4,280,579 to Zaunberger et al.; U.S. Pat. No. 6,182,784B1 to Pestonik; U.S. Pat. No. 6,457,560B1 to Evans et al.; U.S. Pat. No. 6,705,840B1 to Hauser et al.; and U.S. Pat. No. 6,843,056B1 to Langenfeld et al., some of which will be discussed hereinafter. U.S. Pat. No. 2,780,424 merely shows an application for a “T” gearbox to drive a mechanical system, while U.S. Pat. No. 4,280,579 discloses a control for a tracked vehicle steering system comprised of discrete separate components that also include additional gear reducers that are not required in the structures of the present invention due to its usage of low speed, large displacement, high torque gerotor orbital motors in a purely hydrostatic transmission. U.S. Pat. No. 6,182,784 discloses a drive train that uses a “T” gearbox to drive both axles at the same speed whereas, in the structure of the present invention, a “T” gearbox is used to drive two independent hydrostatic pumps, with each pump being coupled with a hydraulic motor whose output shaft functions as an axle to drive a wheel, thus permitting the two axles to rotate at differing speeds and even in opposite directions, if so desired. U.S. Pat. No. 6,457,560B1 pertains to a gearbox oil pumping system associated with a riding mower, showing a plurality of individual oil conduits connecting dual oil pumps to dual fluid motors as well as an oil radiator, an oil reservoir and dual oil filters. U.S. Pat. No. 6,705,840B1 discloses several embodiments of an inline tandem pump apparatus having a pair of coaxially arranged pump shafts and a separate input shaft that is not coaxial with the pump shafts. The pump outputs are connected to remote hydraulic motors. U.S. Pat. No. 6,843,056B1 pertains to a zero turn transaxle comprised of a hydraulic transmission having a pair of hydraulic pumps connected with axially offset hydraulic motors that drive output axles. However, none of these prior art structures teach or suggest the structural and functional features of the present invention.